A virtual device is a logical device, implemented in software, that corresponds to some kind of actual or idealized physical device. There are generally two approaches for modeling virtual devices: the “hardware virtual device” approach which directly models an existing piece of hardware; and the “idealized virtual device” approach which is not a mere reflection of the physical hardware but is optimized for the VM environment.
The hardware virtual device approach offers advantages in regard to compatibility—since the virtual device acts just like a real device in every respect, software that has been designed to interact with that device (e.g., a driver) will work with a hardware virtual device without modification. However, hardware virtual devices are at a disadvantage when it comes to performance—physical hardware is often difficult to emulate with a virtual device without incurring significant overhead costs (and inefficiencies) since hardware designers generally do not take into consideration virtualization issues, and thus hardware virtual devices are often noticeably slower than their real hardware counterparts.
Idealized virtual devices, on the other hand, provide significant freedom for developers to design a virtual device that is both easy to implement and efficient to use. Because the design of an idealized virtual device does not need to conform to limitations imposed by the physical hardware design, idealized virtual devices can be optimized for use within a VM environment. Furthermore, developers of idealized virtual devices do not need to concern themselves with the subtle side effects (such as timing, state changes, etc.) that existing software might rely on for correct operation. Moreover, developers can also create idealized virtual devices that are analogous to hardware that does not in fact exist—for example, a virtual device that allows for communication between a guest system and a host system. However, the downside is that compatibility issues may arise with the idealized virtual device approach since the virtual device may not in fact operate just like the real device in every respect, and software that has been designed to interact with that physical device (e.g., a driver) may not work correctly or at all with an idealized virtual device without modification.
What is needed in the art is an approach to virtualized devices that comprises the advantages of the two existing approaches discussed herein but lacks most of the limitations thereof.